php mailer unreliable

I installed a new server using the Etch Perfect Setup. I have used this guide many times before and have never encountered such a problem.

Since moving to the new server lots of customers are reporting lack of confirmation emails or no reply from our customer service panel.

I run an smtp server on another machine in the same datacentre and this email system is reliable.

So something is wrong with my php mailer on my main server. It isn't a problem with all email destinations, just many. Can anyone suggest things to look for in the configuration files, the dns etc etc because this is a huge problem right now.

Is it possible to setup php to use my authenticated smtp server on the other machine so that I don't have to change all the code for the internal php mailer?